The present invention relates to an electric injection molding machine and a method for controlling injection of the same.
Injection control of an electric injection molding machine for injecting a molding material by the drive force of a servo motor is characterized in that the injection speed and the injection pressure can be controlled with a good response since the servo motor follows control references with a good precision and therefore maintenance of a stable molding precision is possible.
On the other hand, in a hydraulic injection molding machine for injecting a molding material by hydraulics, changes in the temperature of the hydraulic working oil exerts an influence upon the working characteristics of the machine, so the maintenance of a stable molding precision is difficult. Also, the viscoelastic property (compressibility) of the working oil and the characteristics of the hydraulic equipment are not negligible magnitudes, so there are the problems in that nonlinearity is apt to occur between the control references and actual injection speed and injection pressure, so the controllability is poor.
In view of this, there has been a shift from hydraulic injection molding machines to electric injection molding machines.
In order to mold similar molded articles, there are various problems in shifting from hydraulic injection molding machines to electric injection molding machines.
For example, even if setting molding conditions giving a good molding quality set for a hydraulic injection molding machine (injection speed pattern) as they are for an electric injection molding machine, a similar quality is not always obtained. This is because the response characteristics of the actual injection speed and injection pressure with respect to the references differ between hydraulic injection molding machines and electric injection molding machines. For this reason, it was difficult to transplant molding conditions (injection speed pattern) set for a hydraulic injection molding machine to an electric injection molding machine as they are and it was difficult to effectively utilize the molding conditions accumulated in a hydraulic injection molding machine for an electric injection molding machine.
Also, when using the molding conditions set for a hydraulic injection molding machine in an electric injection molding machine and for example injection molding using a mold having a plurality of cavities, there was also the problem that the fluctuations in weight among several molded articles became large.
Further, in an electric injection molding machine, during injection speed control for controlling the injection speed so as to follow references, the injection speed does not slow even when the injection pressure rises when the molding material passes through the gate of the mold. Therefore, most of the molding material heads toward the route through which the material most easily flows in the mold, so the amounts of material filled in the cavities become nonuniform. It is possible to set molding conditions for reducing the variation of weight among a plurality of molded articles in an electric injection molding machine, but it is very difficult to determine such molding conditions. Also, it is necessary to finely set the molding conditions and precisely control the injection speed. Therefore, much time and labor are required for setting the molding conditions.
An object of the present invention is to provide an electric injection molding machine capable of molding an article having a good quality when transplanting molding conditions in a hydraulic injection molding machine to an electric injection molding machine as they are.
Another object of the present invention is to provide a method for controlling the injection of an electric injection molding machine capable of molding an article of a good quality when transplanting molding conditions in a hydraulic injection molding machine to an electric injection molding machine as they are.
Still another object of the present invention is to provide an electric injection molding machine capable of suppressing fluctuations of weight among articles molded by cavities in a mold having a plurality of cavities.
Still another object of the present invention is to provide a method for controlling the injection of an electric injection molding machine capable of suppressing fluctuations of weight among articles molded by cavities in a mold having a plurality of cavities.
According to a first aspect of the present invention, there is provided an electric injection molding machine comprising an electric injection mechanism for injecting a molding material into a mold by a drive force of an electric motor, a servo control means for controlling the injection motor so that an injection speed follows an injection speed reference based on predetermined molding conditions, and a compensating means for compensating the injection speed reference based on characteristics of a hydraulic drive system so that fluctuation characteristics of the injection speed with respect to the injection pressure when controlling the drive of the electric injection mechanism according to the molding conditions approximate the fluctuation characteristics of the injection speed with respect to the injection pressure in a hydraulic injection mechanism for injecting a molding material into a mold by the hydraulic drive system.
According to a second aspect of the present invention, there is provided a method for controlling the injection of an electric injection molding machine comprising a control step for servo control of a drive motor so that an electric injection mechanism for injecting a molding material into a mold by a drive force of an electric motor follows an injection speed reference based on predetermined molding conditions and a compensation step for compensating the injection speed reference based on characteristics of a hydraulic drive system so that fluctuation characteristics of the injection speed with respect to the injection pressure generated in the electric injection mechanism during servo control of the electric motor approximate the fluctuation characteristics of the injection speed with respect to the injection pressure in a hydraulic injection mechanism for injecting a molding material into a mold by the hydraulic drive system.
In the present invention, the injection speed reference is compensated based on the characteristics of a hydraulic drive system so that the fluctuation characteristics of the injection speed with respect to the injection pressure in the electric injection molding machine approximate the fluctuation characteristics of the injection speed with respect to the injection pressure in a hydraulic injection molding machine. For this reason, when controlling the drive of the electric injection molding machine of the present invention under molding conditions for a hydraulic injection molding machine, the fluctuation characteristics of the injection speed with respect to the injection pressure approximate the characteristics of a hydraulic drive system.